This invention relates generally to machine control systems for excavators, and relates more particularly to a method for automatically controlling the depth and angular orientation of an excavator bucket.
Description of the Relevant Art
Excavators are digging machines, typically mounted on tracks. An excavator has a bucket mounted to the end of a two member linkage. One of the links, called a boom, is pivotally mounted to a machine base of the excavator and extends outward in an upward direction. The other link, called a stick, is pivotally mounted at one end to the outer end of the boom and extends downward from the boom pivot. The bucket is pivotally mounted to the outer end of the stick. Three hydraulic cylinders independently move the boom, the stick, and the bucket under the control of an operator or a machine control system. Another hydraulic drive rotates the machine base relative to the track to permit repositioning the bucket for operations like dumping.
Operating an excavator efficiently requires a skilled operator. Each of the couplings between the machine base, boom, stick, and bucket are pivots, so extending or retracting any single hydraulic cylinder or actuator causes the digging edge of the bucket to move in an arc. Most excavating projects, however, involve creating finished surfaces that are planar, either horizontal or sloped. Thus multiple cylinders need to be controlled simultaneously in order to excavate planar surfaces with the bucket. Typically, two joysticks are used by the operator, with each joystick being moveable left and right to control extension and retraction of one cylinder and moveable forward and aft to control extension and retraction of another cylinder.
One problem encountered with an excavator is how to indicate to the operator the depth to which the cutting edge of the bucket is digging so that the correct elevation or grade is obtained by the excavation process. A related problem is that the cutting edge of the bucket can be out of sight of the operator. One known way to indicate depth is to utilize angular sensors that measure the relative angles between the machine base, boom, stick, and bucket, and to calculate the depth of the bucket, using principles of geometry, given the measured angles and the lengths of the links. The calculated depth is then displayed for the operator, as disclosed, for example, in U.S. Pat. No. 4,129,224.
An extension to this concept is to utilize the measured depth and/or slope information to automatically control the movement of the excavator bucket. In U.S. Pat. No. 4,129,224, for example, the hydraulic cylinder that moves the stick is controlled by the operator, and the machine control system automatically controls the boom cylinder and the bucket cylinder to result in a linear movement of the bucket.
A significant shortcoming of such prior automatic machine control systems is that they remove the operator from the control loop when the automatic control is engaged. If the automatic machine control system is automatically controlling the digging depth, but there is a large amount of material to remove before reaching the desired grade, then the machine control system can take too large a cut, which can overload the excavator, resulting in inaccurate cuts and equipment breakage.